.back-to-top {
  position: fixed;
  bottom: 30px;
  right: 30px;
  width: 40px;
  height: 40px;
  border-radius: 50%;
  background: #7B8F00;
  cursor: pointer;
  box-shadow: 0 2px 5px rgba(0,0,0,0.3);
  opacity: 0;
  transition: opacity 0.3s;
  display: flex;
  align-items: center;
  justify-content: center;
  color: white;
  font-size: 24px;
   z-index: 2147483647; /* 浏览器允许的最大层级[7](@ref) */
  mix-blend-mode: difference; /* 与背景色自动反色 */
}
.back-to-top.show {
  opacity: 1;
}